236. Lowest Common Ancestor of a Binary Tree

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree

Example 1:

• Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
• Output: 3
• Explanation: The LCA of nodes 5 and 1 is 3.

Example 2:

• Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
• Output: 5
• Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

Example 3:

• Input: root = [1,2], p = 1, q = 2
• Output: 1

Constraints:

• The number of nodes in the tree is in the range [2, 10^5].
• -10^9 <= Node.val <= 10^9
• All Node.val are unique.
• p != q
• p and q will exist in the tree.

Approach

Solution

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Solution:
    def lowestCommonAncestor(
        self, root: "TreeNode", p: "TreeNode", q: "TreeNode"
    ) -> "TreeNode":

        if not root:
            return None

        if root == p or root == q:
            return root

        l = self.lowestCommonAncestor(root.left, p, q)
        r = self.lowestCommonAncestor(root.right, p, q)

        if l and r:
            return root
        else:
            return l or r